In particular immediately after cold starting of an internal combustion engine, unburnt fuel may be dissolved in a lubricant of the internal combustion engine, then evaporate again as the operating temperature increases. With reciprocating piston gasoline or diesel engines, for example, particularly in the first seconds after cold starting, fuel may condense on the oil film on the cold wall of the combustion chamber and dissolve in the oil film. This problem arises mainly when fuel is injected directly into the combustion chamber and particularly with gasoline engines, but also with other fuel delivery methods and internal combustion engines.
The dissolving of fuel in the lubricant causes an undesirable change in the lubricating properties of the lubricant, thereby possibly increasing wear and the probability of the occurrence of a malfunction, and reducing the life expectancy of the internal combustion engine.
The fuel dissolved in the lubricant evaporates again as the operating temperature increases and collects in a reciprocating piston engine mainly in the crankcase. In order to prevent emission of unburned fuel into the environment, the crankcase is connected to the intake tract via a crankcase breather. Because of a pressure drop from the crankcase to the intake tract, there arises a mass flow from the crankcase into the intake tract which is dependent on the operating state of the internal combustion engine. Said mass flow (known as blow-by) consists of exhaust gas and air which are fed from the combustion chamber past the piston rings into the crankcase and possibly fuel which is evaporated out of the lubricant in the crankcase.
The control system of a modern internal combustion engine monitors the operability of its components by performing diagnostics on the available operating parameters. Fuel evaporated out of the lubricant and entering the intake tract via the crankcase breather riches the fuel-air mixture in the combustion chamber or chambers of the internal combustion engine. For complete combustion of the fuel and of the atmospheric oxygen (λ=1), the internal combustion engine's control system must meter in less fuel relative to the fresh air supplied to the internal combustion engine. Such an anomaly is interpreted by the control system as an engine malfunction, e.g. on the fuel supply device or on a lambda sensor. In order to avoid this misinterpretation, conventionally an excessively low amount of fuel to be metered into the internal combustion engine within a predetermined time interval after a cold start is not interpreted as a fault. This significantly limits engine malfunction diagnostics. This limitation is particularly severe if the engine is always operated only for a short time, e.g. in city traffic.